The present invention relates to an improved method of manufacturing a composite railroad cross tie, and more particularly a railway cross tie characterized by a core which is encased in plastic or resin.
Conventionally, railway or railroad cross ties or xe2x80x9csleepersxe2x80x9d have been formed from hardwood logs. The logs are cut into an elongated rectangular shape and typically have a width of between about 8 and 10 inches, a height of 6 to 8 inches, and a length of between about 7 and 9 feet. In use, the cross ties are positioned resting on a concrete rail bed or partially submerged within ballast such as crushed gravel or rock. Between about 20 and 40 cross ties are used to support each rail section of railroad track by driving spikes into the cross ties so that the spike heads engage a lower flange on each rail. Conventional hardwood railroad cross ties present disadvantages in that given the scarcity of hardwoods they are expensive to produce and susceptible to decay.
To prolong the life-span of hardwood railroad cross ties, it is known to coat or paint the sides and ends of hardwood cross ties with preserving chemicals, such as coal tar creosote or the like in an effort to delay their deterioration. The use of creosote as a preservative, suffers the disadvantage that it is a toxic substance and a suspected carcinogenic. Creosote coated cross ties therefore result in potential environmental hazards both in the initial coating of the cross ties, and through the possibility of the creosote leaching into the surrounding soil or water table.
When chemically treated with preservatives, hardwood cross ties will have a typical life span ranging from a maximum of about fifty years where optimum conditions and drainage occur, to as low as two years in high humidity environments. Even when cross ties are treated with decay inhibiting chemicals, the chemical preservatives will typically only penetrate between 2 and 10 mm into the ends and exterior surfaces of the cross tie. As the hardwood dries, it differentially shrinks with age. As a result of wood shrinkage, it is common for deep cracks or checks to form in the sides of cross ties and which may extend up to two inches into the cross tie surfaces. These cracks or checks in turn permit water and insects to reach the untreated interior portion of the wood, speeding the cross tie decay.
When repairing a rail section to replace cross ties which have deteriorated, it is often the practice to replace all of the cross ties along the entire rail section, regardless of whether or not even the majority of the hardwood cross ties may have deteriorated to such an extent as to be in need of replacement. Chemically treated hardwood railroad cross ties suffer a further disadvantage in that the toxic chemicals present a disposal difficulty for discarded cross ties, given environmental concerns over the hazardous chemical preservatives. As a result, it is frequently necessary to not only pay for new railroad cross ties, but also to pay a surcharge for the disposal of each railroad cross tie which is replaced.
In an attempt to overcome the disadvantages associated with conventional hardwood railroad cross ties, various individuals have proposed concrete, composite and manufactured cross tie constructions for use in place of hardwood logs. Concrete cross ties are very heavy, weighing as much as three times that of a hardwood cross tie, and are expensive to install. As well, concrete cross ties have a tendency to crack, and also take a heavy toll on the moving rail cars and cargo due to their lack of energy absorbing characteristics.
U.S. Pat. No. 4,150,790 to Potter, which issued Jun. 20, 1995 discloses a steel beam reinforced lignocellulosic cross tie. U.S. Pat. No. 4,083,491 to Hill, which issued Aug. 18, 1975 discloses a cross tie formed from two end blocks which are joined by a pair of metal sides. The manufactured sleepers or cross ties of Hill and Potter have not yet achieved commercial success as they are expensive to manufacture, and further they do not address the difficulties associated with the disposal of the millions of existing creosote impregnated hardwood cross ties which are currently in use.
U.S. Pat. No. 3,416,727 to Collins, which issued Dec. 17, 1968 discloses a laminated railroad tie formed from a shredded hardwood filler and synthetic resin made from waste wood. Collins suffers a disadvantage in that in addition to the added expense of manufacture, the use of shredded wood fiber may in fact increase the degradation of the cross tie. Plastic cross ties are very expensive, with the result that their use is restricted to areas which are difficult to access, such as tunnels, which are one of the most expensive areas for replacing cross ties and which offsets the high initial cost of plastic cross ties. Plastic cross ties are also usually made from polyolefine compounds which tend to stretch or elongate and creep under the heavy loads, particularly at elevated temperatures, which restricts their use. Like Hill and Potter, plastic based cross ties such as those proposed by Collins also do not address the problem of disposal of existing hardwood cross ties.
It has been proposed to provide a composite cross tie which consists of an inner core material of natural or engineered wood which is completely encased in an outer plastic shell. The inventors have appreciated, however, potential difficulties in the manufacture of coated core members. To ensure consistent finished cross tie dimensions, suggested methods of manufacture would involve injection molding the coating about each core. Injection molding is cost intensive from an equipment and tooling point of view. This process also shows relatively long cycle times in manufacturing due to the relatively thick layer of plastic needed to encapsulate the core.
The present invention overcomes at least some of the disadvantages of prior art railroad cross ties by providing a composite cross tie having an inner core member encased in plastic or resin. The coating layer is applied to the inner core by passing the core member through an enlarged die head which is configured to form an extrusion coating about the core in substantially the desired finished dimension.
Another object of the invention is to provide an improved method of forming a composite railroad cross tie to permit its manufacture quickly and economically in a continuous extrusion process.
The ends of the ties can be coated by resin provided in the extrusion process or by separate end caps either molded prior to the extrusion process or after the extrusion process.
The present invention also seeks to overcome the disadvantages associated with the prior art by providing an improved railroad cross tie or xe2x80x9csleeperxe2x80x9d, which has a water impermeable outer coating or shell to provide enhanced resistance to decay.
Another object of the invention is to provide a composite railroad cross tie which is characterized by an inner core member of natural or engineered wood, and extruded outer plastic coating layer or a pair of end covers or caps, which together with the coating layer substantially isolate the core from moisture and/or insects which may otherwise speed its decay. These end caps can be added to the core before or after the extrusion process.
Another object of the invention is to provide a railway or railroad composite cross tie which may be easily and economically manufactured, and which has a core formed from new or recycled hardwood, engineered woods, concrete, plastic composites or other such structurally suitable materials.
Another object of the invention is to provide a composite railroad cross tie which has substantially the same dimensions as a conventional creosote treated hardwood cross tie, so as to facilitate the replacement of worn hardwood cross ties partially submerged within rail bed ballast.
A further object of the invention is to provide an improved method of manufacturing a composite railroad cross tie having substantially the same stability, weight and physical properties as a new conventional hardwood cross tie.
Another object of the invention is to provide a composite railroad cross tie having a rectangular wooden interior core member which is enveloped at each of its ends by extruded plastic or resin or a separate end cover or cap, and along its longitudinal length by an outer coating of thermoplastic, thermosetting resins and/or rubbers or mixtures thereof, and in which the coating has a thickness selected so as not to interfere with the insertion and gripping of a conventional rail spike into the inner wooden core member.
The present invention provides a composite railroad cross tie which is characterized by an elongated core member, one and preferably two end covers or caps and a coating layer or shell. The end caps may be formed from a number of materials such as plastic, resins, metals, glass, as well as composites or mixtures thereof, and the outer coating layer is most preferably a thermoplastic or thermosetting resin.
Preferably, the core member is formed from new, recycled or engineered wood and is completely encased or enveloped by the end caps and outer coating, so as to be substantially sealed thereby from moisture, the atmosphere and insects. The core may be formed from hardwood or alternately engineered man-made wood products including by way of non-limiting examples plywood, micro laminates, oriented strand board and the like. The inner wooden core member preferably has a generally rectangular shape, however, other core profiles are possible.
The end covers are positioned over each end of the core member with the outer coating layer provided along the length of the core member to substantially encase the core member and provide the railroad cross tie with the desired dimensional profile.
In manufacture, the core member is sized to a dimension which is less than the desired dimension of the finished composite cross tie. The end caps are then positioned over ends of the core member. Following the positioning of the end caps, the coating is applied to the core member in a continuous process by passing a series of core members with their end caps in a substantially end-to-end configuration through a shaping die and most preferably, a cross head die. As the core members are moved through the crosshead die, the coating material is applied thereto in a sufficiently liquid form to infill any nail or spike holes, cracks which may exist in the core material. The outer coating layer is extruded over the core with a thickness of at least about 0.4 mm, and most preferably between about 1 and 10 mm, so as not to interfere with the driving of a conventional rail spike therethrough into the core member.
Most preferably, the cross head die is configured so that the extruded composite railroad cross tie has an overall dimension and shape substantially corresponding to a conventional hardwood railroad cross tie.
Following emergence from the cross head die, the composite railway ties are separated from each other by breaking or cutting the extruded ties at the point of contact between the abutting end caps.
In a preferred embodiment, the core member is formed from recycling a discarded hardwood railway tie, and preferably a discarded railway tie which was originally treated with creosote or other wood preserving chemicals. To form the core member, the discarded railway tie is refurbished by reducing its size on all sides to expose fresh wood surfaces. The chemically treated side surfaces of the hardwood tie are removed by a saw, sanding, planing or other suitable means to the required depth in most cases between about 0.4 and 20 mm, allowing for the additional layer of coating material. In addition, material may be removed from each end of the discarded cross tie in most cases to a depth of between about 1 and 100 mm, since penetration of preserving chemicals and destruction by natural causes is typically greater at the cross tie ends.
The end caps most preferably have a peripheral dimension which substantially corresponds to that of the refurbished cross tie ends. Although not necessary, the end caps may optionally be secured to the ends of the core member by the use of adhesives by separate mechanical fasteners such as nails, clips, screws and the like, or either in a friction fit by the engagement of a mechanical fastening element integrally formed as part of the end cap.
The outer coating layer may be selected from a number of water impermeable compounds including thermoplastics, thermosetting resin, rubbers and mixtures thereof. The use of polyolefins, such as polyethylene as an outer coating is highly advantageous as these coatings will permit some natural expansion and contraction of the inner wooden core without splitting, and thereby maintain the wooden core member substantially sealed from the environment.
Accordingly, in one aspect the present invention resides in a generally rectangular composite railroad cross tie for use in replacing a conventional hardwood cross tie comprising,
an inner wooden core member, said inner core member having a pair of end surfaces longitudinally elongated side surfaces extending from a first one of said end surfaces to the other second one of said end surfaces,
a first end covering member secured to and substantially covering said first end surface,
an outer coating layer substantially bonded to said side surfaces of said core member, said outer coating layer comprising plastic and having an approximate average thickness selected preferably at between about 0.4 mm and 20 mm,
wherein said composite railroad cross tie has an overall dimension substantially corresponding to that of said conventional hardwood cross tie.
In another aspect the present invention resides in a composite railroad cross tie for supporting railroad track rails on a ballast or a concrete rail bed, comprising
a generally rectangular wooden core member having longitudinally elongated side surfaces extending from a first member end to a second member end,
a pair of end cap members each having a substantially modular construction and a complementary size to a corresponding one of said first and second ends, a first one of said end cap members being secured to said first end, and the second other one of said end cap members being secured to the second end,
an outer coating layer substantially bonded to said side surfaces of said core member, said outer coating layer having an average thickness preferably of between about 0.4 mm and 20 mm and being selected from the group consisting of a thermoplastic, a thermosetting resin, and mixtures thereof, wherein said end cap members and said coating layer substantially seal said core member from the atmosphere.
In a further aspect the present invention resides in a method of manufacturing a composite railroad cross tie from a discarded hardwood railroad cross tie which has chemically treated side and end surfaces, a method of manufacturing a composite railroad cross tie using an extrusion die having an axially extending feed bore, a generally rectangular die opening aligned with said feed bore, and an extrudate distribution passage communicating with said die opening, the railroad cross tie characterized by:
a generally rectangular inner wooden core member having a longitudinally elongated side surface extending from a first core member end to a second core member end,
an end cap member in sealing engagement with said first core member end, and
a coating layer substantially bonded to said side surfaces,
wherein said cross tie is formed by:
securing said end cap member to said first end, and
moving said core member together with said end cap member axially through said feed bore and past said die opening while extruding molten extrudate from the distribution passage into the die opening and about the side surfaces of the core member, and
wherein said extrudate is selected from a thermoplastic, thermosetting resin and mixtures thereof.